Mixing up Chicks... how old?

Hi

I am looking at getting 3 different female chicks (approx 10-12 weeks old) for my backyard.
There will be hopefully be 2 proper breed chickens (Araucana and possibly Blue Leghorn), the third a standard Brown Shaver (which the commercial layers use locally). The breed chickens will come from a mixed flock, the Shaver from a flock with only shavers.
The lady selling the breed chickens is concerned that the Shaver will attack the Araucana and Leghorn, because the Shaver will not be used to different coloured chickens (even though they will all be only 10-12 weeks old).
Is this likely to be an issue at that age? From what i've researched, none of these breeds are particularly aggressive.

I really want one very very good layer, and then mix it up a bit with the other two for different egg types. My understanding is that Leghorns are good layers, and Araucana medium layers...?

They are likely to live in a relatively good sized coop, being let out from time to time, but not roaming free (we don't have enough safe space for that).

Please give me your thoughts!

There is always an integration period when introducing new birds to each other and there will certainly be pecking order scuffles between all the birds to establish their flock order. Are the first two birds that you are getting from the breeder going to be brooded and raised together? If not there is just as much likelyhood of them attacking each other as your shaver attacking them. If they are indeed raised together and familiar with each other already then you can fence off an area for the one new bird so they can get used to each other through a fence for a week or so. SInce they are young it may not take long, just have to see how it goes. At the same time, introducing a single bird to any small flock, even a flock of only two, can be difficult. Every situation is different, sometimes it goes smoothly sometimes it's harder, just have to take it a day at a time and see how it goes.